A conventional optical communication system is illustrated in FIG. 1. The optical communication system 1 comprises a transmitter 10, an optical medium 20 (e.g., an optical fiber, a waveguide, free space, etc.), and a receiver 30. The optical communication system 1 receives a data input on connection 5 and generates a data output that is applied on connection 35. The transmitter 10 includes a data input 12 and an optical output 14. The receiver 30 includes an optical input 32 and a data output 34. The optical medium 20 has a first end 22 that is coupled to the optical output 14 and a second end 24 that is coupled to the optical input 32. The transmitter 10 receives data in an electrical format and couples an amplitude-modulated optical representation of the data on the optical medium 20. The receiver 30 receives the amplitude modulated optical representation of the data from the optical medium 20 and converts the same to an electrical representation of the received data.
The optical modulation amplitude (OMA) of a data signal is an important parameter that is used in specifying the performance of optical links used in digital communication systems. At a given receiver noise floor, the OMA directly relates to the bit error ratio (BER) of a communication system.
In bipolar non-return to zero (NRZ) optical signaling schemes, only two discrete optical power levels are used. The higher level or PH and the lower level or PL. FIG. 2 includes a plot 200 of optical power versus time for both the transmitter 10 and the receiver 30 of FIG. 1. As illustrated in FIG. 2, OMA is defined as the difference between the high and low power levels, which can be represented mathematically as:OMA=PH−PL   Equation 1Average signal power is simply the average of the high and low power levels, i.e.,
                              P          AVG                =                                            P              H                        +                          P              L                                2                                    Equation        ⁢                                  ⁢        2            The extinction ratio (ER) is the ratio between the high and low power levels:
                    ER        =                              P            H                                P            L                                              Equation        ⁢                                  ⁢        3            From Equation 1, Equation 2 and Equation 3, the following relationship can be derived:
                    OMA        =                  2          ⁢                                          ⁢                                    P              AVG                        ⁡                          [                                                ER                  -                  1                                                  ER                  +                  1                                            ]                                                          Equation        ⁢                                  ⁢        4            
OMA and ER by themselves are relative quantities, since they specify the difference and a ratio of power levels, respectively. To derive an absolute quantity from the OMA or ER an additional point of reference, such as PAVG, PH, or PL, is required. Each of the relationships defined in Equations 3 and 4 depend on one of these additional points of reference.
For example, an OMA of 100 μW can correspond to an infinite number of possible values for PAVG, PH, or PL. PH could be 100 μW with PL equal to 0 μW, or PH could be 150 μW with PL equal to 50 μW, or PH could be 100 mW with PL equal to 99.9 mW, etc.
In the alternate case of ER, a similar example using an ER=10 can correspond to an infinite number of possible values for PAVG, PH, or PL. PH could be 100 μW with PL equal to 10 μW, or PH could be 150 μW with PL equal to 15 μW, or PH could be 100 mW with PL equal to 10 mW, etc.
If in addition to OMA and ER a reference point of PAVG=100 μW is specified, then the ambiguity has been removed. With an OMA of 100 μW and PAVG=100 μW, PH can only be 150 μW and PL can only be 50 μW. If the ER is 10 and PAVG=100 μW, then PH can only be 182 μW and PL can only be 18.2 μW.
While it may seem apparent that OMA and ER are nearly equivalent, there are differences. One of these differences is how OMA and ER change as a signal propagates through an optical communication system. Assuming an optical communication system with linear attenuation between two points, the ER will stay constant as the signal is attenuated, while the OMA will change by a factor equal to the attenuation. For example, over 10 km of optical fiber with an attenuation of 0.3 dB/km, the total attenuation over 10 km is 3 dB, which is equivalent to a factor of 2. A signal transmitted through the optical fiber that starts with PH of 1 mW and PL of 0.1 mW, has an ER of 1/0.1=10 and an OMA=1−0.1=0.90 mW at the input to the optical fiber. At the output of the optical fiber, PH is 0.5 mW and PL of 0.05 mW (both are reduced by a factor of two). Therefore, ER is 0.5/0.05=10 and OMA=0.5−0.05=0.45 mW. Thus, ER is the same and OMA is reduced by a factor of two. Once the ER is known, an average power measurement from anywhere in the optical communication system will yield enough information to calculate PH, PL and OMA. On the other hand, a measure of OMA at any point in the system does not provide enough information to determine the OMA at another point in the system without knowing the magnitude of the attenuation or measuring additional parameters (such as PAVG, PH, or PL).
To optimize BER performance of an optical communication link, the OMA should be as large as possible. In optical communication links there are upper and lower limits on PAVG and OMA. In an optical receiver, there is an upper limit on the optical power that can be received. When the received optical power exceeds this upper limit, saturation effects degrade BER performance For optimum receiver BER performance, the OMA should be as large as possible while avoiding the upper power limit, which occurs when PL is zero and PH is just below the upper power limit. For optical transmitters that use a laser as a light source, it is difficult to reduce PL to zero. When a laser is switched from a completely off state to an on state, turn-on delay and relaxation oscillation negatively affect the communication link. If the laser is biased above its threshold level so that it is always on, problems with turn-on delay and relaxation oscillation decrease. For this reason, practical laser transmitters emit some optical power at PL. A complicating factor is that the laser threshold changes significantly with temperature, making it difficult to keep the difference between the bias and the threshold constant. Precise control of the bias current over a large temperature range adds significant complexity and cost to optical transmitters.
For conventional optical communication links that use relatively low-loss multimode fiber as the communication medium, a combination of the ER and the average power at the transmitter has provided an adequate measure of communication link quality. For optical communication applications that use large-core fiber (e.g., polymer optical fiber (POF)) the combination of ER and average power at the transmitter does not provide an adequate measure of optical communication link quality. While POF is inexpensive and easy to terminate with common tools and ordinary polishing paper, POF attenuates more and provides less bandwidth when compared to an optical fiber of similar length made from silica. Communication links using POF have been used in industrial control applications, robotics, and automotive applications where signaling rates are much lower than those used in high-speed telecommunication applications. The relatively low signaling rates, which enable simple and inexpensive light-emitting diode (LED) based transmitters, has proved to be a significant factor in market acceptance and penetration for POF communication based systems. However, there is a demand in industrial automation applications to use the Fast Ethernet data transfer protocol (100 Mbps) over POF links up to 50 meters long and hard cladded silica (HCS) links up to 100 meters long. Beyond these distances, the bandwidth of standard 0.5 numerical aperture (NA) POF and 0.37 NA HCS links will not support Fast Ethernet communications. The limited bandwidth of POF and HCS communication links, even at the desired maximum distances, renders the combination of ER and the average power at the transmitter ineffective as a measure of communication link quality. This is because the average received light power can be nominal but modal dispersion in the communication medium may reduce the difference between the high and low signal levels at the receiver. Such a reduction in the difference between the high and low signal levels can severely degrade BER performance of the communication link.